1. Field of the Invention
The present invention relates to a high voltage power supply used for an image forming apparatus using an electrophotographic system, and the image forming apparatus.
2. Description of the Related Art
An image forming apparatus adopting an electrophotographic system includes various high voltage power supplies, which are necessary for an image forming process for forming an image on a recording medium. This high voltage power supply may be, for instance, a high voltage power supply for a charge apparatus, a high voltage power supply for a developing apparatus and a high voltage power supply for a transfer apparatus, which are various modularized power supplies. These modularized power supplies have specifications different according to the configurations of image forming apparatuses. For instance, there are specifications that superpose the output of an ac high voltage power supply on the output of a dc high voltage power supply, and specifications that superpose the output of a dc positive high voltage power supply on the output of a dc negative high voltage power supply. Furthermore, there are various specifications on specified voltage, specified current and constant current control systems, constant voltage control systems, single value output and multistage value control output, and load conditions.
For instance, as to a transfer voltage, to remove toner attached to a transfer roller during a cleaning operation of an image forming apparatus, it is required to apply, on the transfer roller, a voltage having a polarity opposite to the polarity of a voltage applied during normal transfer. In general, toner itself has a negative polarity. Accordingly, during a transfer process of forming an image, the toner is transferred from a photosensitive drum to an intermediate transfer belt and from the intermediate transfer belt to a recording medium by applying a voltage from a high voltage power supply having a positive polarity (hereinafter, referred to as “positive power supply”). In contrast, during a cleaning process, toner is discharged from a transfer member to the intermediate transfer belt, and toner that is on the intermediate transfer belt and to be removed is transferred via the photosensitive drum into a used toner container. For instance, Japanese Patent Application Laid-Open No. 2008-309904 proposes an image forming apparatus that inversely transfers toner that has not been transferred on a recording medium and remains on an intermediate transfer belt onto a photosensitive drum and collects the toner.
FIG. 8 schematically illustrates a conventional secondary transfer power supply and ITB cleaning power supply in an image forming apparatus including an intermediate transfer belt (hereinafter, abbreviated as “ITB”) 900. In FIG. 8, an MPU 901 is a microprocessor that detects load currents flowing through a secondary transfer roller 906 and an ITB cleaning brush 916 and controls outputs of the secondary transfer power supply and the ITB cleaning power supply based on the detected load current value. A secondary transfer positive power supply 902 and a secondary transfer negative power supply 903 apply positive and negative voltages, respectively, on the transfer roller 906 according to control signals (analog signals) from the MPU 901 via a D/A port. An ITB cleaning positive power supply 912 and an ITB cleaning negative power supply 913 apply positive and negative voltages, respectively, on the ITB cleaning brush 916 according to control signals (analog signals) from the MPU 901 via a D/A port. Resistors 904, 905, 914 and 915 are bleeder resistors.
In image forming, a toner image on a photosensitive drum 908 is transferred onto the ITB 900 by a transfer voltage applied from a primary transfer power supply (not illustrated) to a primary transfer pad 909. Subsequently, according to a control by the MPU 901, a transfer voltage having a positive polarity is applied from the secondary transfer positive power supply 902 to the secondary transfer roller 906, and the toner image on the ITB 900 is transferred onto the recording medium 920. The toner that has not been transferred onto the recording medium 920 and remains on the ITB 900 is temporarily collected to an ITB cleaning brush 916 by the MPU 901 causing the ITB cleaning positive power supply 912 to apply a voltage having the positive polarity to the ITB cleaning brush 916. At this time, a secondary transfer current having flown to the secondary transfer roller 906 flows to the ground (hereinafter, referred to as “GND”) of the photosensitive drum 908, and passes, from the GND of a load current detection circuit 907, through this load current detection circuit 907 and bleeder resistor 905, and returns to the secondary transfer positive power supply 902. Accordingly, the value of the secondary transfer current can be detected by the load current detection circuit 907. The detected load current value is input into the MPU 901 via an A/D port according to an analog signal output from the load current detection circuit 907. Likewise, the ITB cleaning current flows from the ITB cleaning brush 916 to the GND of the photosensitive drum 908, passes from the GND of the load current detection circuit 917 through this load current detection circuit 917 and the bleeder resistor 915, and returns to the ITB cleaning positive power supply 912. Accordingly, the value of the ITB cleaning current can be detected by the load current detection circuit 917. The detected load current value is input into the MPU 901 via an A/D port according to an analog signal output from the load current detection circuit 917.
Meanwhile, in the cleaning process, the MPU 901 turns on the secondary transfer negative power supply 903 and the ITB cleaning negative power supply 913. The negative voltages supplied from the respective negative power supplies are applied to the secondary transfer roller 906 and the ITB cleaning brush 916 via the bleeder resistors 904 and 914. The toner adhering to the secondary transfer roller 906 is transferred to the ITB 900 by the negative voltage applied to the secondary transfer roller 906, and removed from this secondary transfer roller 906. The toner temporarily collected and accumulated at the ITB cleaning brush 916 is discharged onto the ITB 900 by the negative voltage applied to the ITB cleaning brush 916. The discharged toner is inversely transferred to the photosensitive drum 908 and collected into a cleaner container in this photosensitive drum.
According to the power supply configuration superposing the positive power supply on the negative power supply as illustrated in FIG. 8, a negative voltage is output via the bleeder resistor, thereby reducing the accuracies of the output voltage and output current. However, in general, a negative voltage applied for removing toner does not require strict voltage and current accuracies in comparison with a positive voltage applied for forming an image. Accordingly, only application of a voltage having at least a predetermined value can satisfy a required cleaning performance.
In recent years, further reduction in size and cost of an image forming apparatus has been required. As described above, the high voltage power supply included in the conventional image forming apparatus is provided with independent power supply circuits for respective voltages to be applied. This configuration causes a problem of increase in cost due to increase in the number of components and a problem of increase in area of a circuit substrate. Accordingly, also the high voltage power supply is required to be reduced in size and cost.